There have been publicly known various research reports or patents on quality monitoring of a welded product, automatically, non-distructively and/or simultaneously with completion of the welding operation.
Among them, a monitoring system referring to the relative displacement of two confronting electrodes is generally admitted to be more practical than others. (The relative displacement takes places due to expansion, shrinking and/or indentation of the welded portion or nugget).
Therefore, a number of patents and literature articles are known as prior art which are basically intended to monitor or to control the weld quality by utilizing a maximum value of the relative displacement (during one cycle of welding operation) or the rate of the displacement.
These prior art methods are classified in three categories as follows:
1. A METHOD OF COMPARING THE MAXIMUM AMOUNT OF THE DISPLACEMENT WITH A PREDETERMINED STANDARD (U.S. Pat. No. 2,851,584), or of controlling welding current so that said amount of the displacement shall fall into a predetermined value (U.S. Pat. No. 3,569,659),
2. a method of comparing the rate of the displacement at an intial stage of one welding operation cycle with a predetermined standard rate (Welding Journal, April -65 "Electrode Separation Applied to Quality Control in Resistance Welding", U.S. Pat. No. 3,400,242, Welding Research International Vol. 2, No. 1, 1972), and
3. a method of controlling welding current so that the relation between time and electrode displacement shall coincide with a predetermined characteristic curve (U.S. Pat. No. 3,404,252).
Having both merits and demerits, however, these prior inventions cannot be practically employed for fabrication, such as automobiles, in which the "splashing" may be accepted under certain conditions as described hereinafter.
For example, in an assembly line of an automobile manufacturing factory, thousands of welding units are usually worked for a unit car, inevitably resulting in a voltage drop due to simultaneous operation of welding units and consumption of electrode tips (i.e., increasing of the tip diameter). Furthermore, there are operated a number of welding machines of the so called series type in which a shunting current, i.e., a current ineffective to welding often occupies as much as 30% of the total current. For these reasons the practical welding current shall be comparatively larger than the standard specification recommended by RWMA (Resistance Welding Machine Manufacturers' Associatiqn). In spite of such a large current being used, strength of a welded joint may be unexpectedly reduced when some of the disadvantageous factors affect together at a time. Therefore, it is very important to establish a reliable method of judging or determining weld qualities in order to prevent the aforesaid shortcomings of resistance spot welding.
Particularly, reliable methods and apparatus have been desired to take into consideration the occurance of splash to which such conditions as mentioned above are very susceptible.